parser options

Thanks for this tool. I was attempting to write a simple Buble compiler for Wallaby.js, but was running into trouble. I would need to pass locations: true into Acorn, but Buble's transform() does not allow for this.

I was able to work around this issue by writing a preprocessor instead, which is not as performant, but a nice one-liner to generate inline coverage:

{'**/*.js':file=>buble.transform(file.content,{source:file.path})}

Not a big deal given the workaround, but was wondering if you would be open to allowing granular control over the parser?